Control valves for hydraulic systems, and particularly when they are intended for operation with highly pressurized hydraulic liquids have problems with respect to sealing and overall weight. Sealing by means of rubber or rubbery sealing rings is difficult to maintain for an extended period of time when the system operates under high pressure, the seals being particularly difficult to maintain between relatively movable parts. High precision, interengaging control elements with tight tolerances can be made; such relatively movable elements, however, are very expensive to manufacture and even the smallest contaminants or particles within the hydraulic fluids can interfere with operation of tightly fitting relatively movable elements.